Lean-burning engines, or engines that run on an air/fuel mixture with a stoichiometrically greater amount of air than fuel can offer improved fuel economy relative to engines configured to run on stoichiometric air/fuel mixtures.
However, lean-burning engines also may pose various disadvantages. For example, burning a lean air/fuel mixture may decrease the reduction of nitrogen oxides (collectively referred to as “NOx”).
Various mechanisms have been developed to reduce NOx emissions in lean-burning engines. One mechanism is a catalyst known as a NOx trap. The NOx trap is a catalytic device typically positioned downstream of the catalytic converter in an emissions system, and is configured to retain NOx when the engine is running a lean air/fuel mixture for eventual reduction when the engine runs a more rich air/fuel mixture. A typical NOx trap includes an alkali or alkaline metal, such as barium or calcium, to which NOx adsorbs when the engine is running a lean air/fuel mixture. The engine can then be configured to periodically run a richer air/fuel mixture to produce carbon monoxide, hydrogen gas and various hydrocarbons to reduce the NOx in the trap, thus decreasing NOx emissions and regenerating the trap.
The use of a NOx trap can substantially reduce NOx emissions from a lean-burning engine. However, NOx traps are also susceptible to poisoning from sulfur in fuels. Sulfur, typically in the form of sulfate (SO42−) may adsorb to the trap in the form of a material such as barium or calcium sulfate. The sulfur compounds may block NOx from adsorbing to the trap surfaces, and thus may increase NOx emissions.
Various methods of desulfating NOx traps may be used. These methods, while effective in removing SOx from the trap surfaces, can cause the production of hydrogen sulfide. Specifically, the inventors herein have recognized that variations in air-fuel ratio that may occur at certain stages of reactions can cause the production of hydrogen sulfide in different ways depending on exhaust temperature.